The present invention relates to polymer gel electrolytes for use in lithium secondary batteries, lithium secondary batteries incorporating the electrolyte, and binders for use in lithium secondary batteries.
In recent years, there is a growing demand for precision electric or electronic devices which are small-sized and suitable for portable use, such as audio tape recorders, camera-incorporating video tape recorders, personal computers and cellular phones. This trend entails a further demand for so-called secondary cells or batteries which are compact, lightweight and rechargeable and have a high energy density for use in these devices as drive power sources. New secondary batteries of high performance including nickel-hydrogen batteries and lithium batteries have been made commercially available in addition to the conventional lead storage batteries and nickel-cadmium secondary batteries.
Much has recently been expected of high-performance secondary cells or batteries for the amelioration of environmental problems such as the reduction of CO2 emissions and air pollution control. Stated more specifically, electric vehicles (EVs) and so-called load conditioners for the purpose of leveling off the load of power supply are expected to be promising. EVs are adapted to obtain powder mainly from the electric energy stored in secondary batteries. It is not too much to say that the performance of the EV is dependent on the energy density of the secondary battery installed in the vehicle. The load conditioner is designed to accumulate excessive electric power of nighttime in secondary batteries, which are discharged for use in the daytime in the event of a power shortage. Such load conditioners need to be installed in individual buildings and must have a high energy density because the space available is limited.
Characteristic of these uses is that high-performance secondary batteries of large size are generally required. A high safety level is also required of high-performance batteries of great size because of the large quantities of energy stored therein.
Among the new secondary cells or batteries, expectations are great for lithium secondary cells or batteries wherein the negative electrode comprises metallic lithium, lithium alloy, or a compound capable of absorbing and desorbing lithium ions because of their high energy density.
Secondary batteries generally comprise a negative electrode, positive electrode, electrolyte (liquid electrolyte) having ionic conductivity and separator for preventing short-circuiting between the negative and positive electrodes. Lithium secondary batteries have incorporated therein a nonaqueous electrolyte, i.e., solution of a lithium salt in a carbonic acid ester or like organic solvent. The nonaqueous electrolyte is substantially inflammable and therefore involves the hazard of burning or explosion at all times, hence the importance of countermeasures for assuring safety.
Especially, the separator serves to prevent short-circuiting between the negative and positive electrodes, also has the function of holding the electrolyte within the battery system with good stability and is accordingly the most important component in ensuring safety.
The separators which are most prevalently used at present are porous membranes of a hydrocarbon polyolefin resin such as polyethylene or polypropylene. These membranes have the so-called shutdown function of closing the pores when the resin melts at a high temperature of not lower than the melting point, and are capable of preventing the rupture of the battery due to an abnormal reaction. Such polyolefin resins nevertheless still remain to be improved in their ability to retain the electrolyte and are likely to permit the electrolyte to seep out of the battery can, hence the hazard of electrolyte leakage.
Proposals have been made of rendering the surface of polyolefin resin hydrophilic as by a treatment with plasma, and improving the electrolyte retentivity with use of a surfactant, whereas these methods have yet to be improved in effectiveness.
Although attempts have been made to dispense with liquid electrolytes, that is, to use a solid electrolyte in place of the liquid electrolyte, solid electrolytes have the drawback of failing to afford a great discharge current due to low electric conductivity and have not been placed into actual use.
Accordingly, attention has been directed to so-called xe2x80x9cpolymer gel electrolytesxe2x80x9d which are improved in electrolyte retentivity by incorporating as a separator a resin which swells in a nonaqueous liquid electrolyte. The characteristics required of polymer gel electrolytes include:
(1) high ability to retain the liquid electrolyte and high lithium ion conductivity,
(2) being readily available in the form of a thin membrane which has sufficient strength for use in the battery,
(3) chemical stability, especially high stability against oxidation, in the battery reaction system, and
(4) thermal stability for use at high temperatures.
Although extensive research is presently conducted on polyethylene oxide, polypropylene oxide and like polyether resins for use as polymer gel electrolytes, these resins are not fully satisfactory in the characteristics (1), still remaining to be improved in battery performance and safety.
Fluorine-containing polymers are generally outstanding in chemical and thermal stabilities and are thought to be of high potential ability as gel electrolytes fulfilling the requirements (3) and (4).
For example, U.S. Pat. No. 5,418,091 proposes a copolymer of vinylidene fluoride (2F) and hexafluoropropylene (6F) for use as a gel electrolyte. The process disclosed in this patent affords a gel electrolyte which is stable in the battery system. However, the 2F/6F copolymer varies greatly in physical properties depending on the proportion of 6F. For example, a high 6F proportion provides high electrolyte retentivity and high ionic conductivity but gives impaired strength to the membrane, which becomes partly dissolved in the electrolyte in an extreme case. In fact, we have found that when having a 2F/6F ratio of 78/22 in mole ratio, the copolymer dissolves in an electrolyte (mixture of equal portions of propylene carbonate (PC) and ethylene carbonate (EC)). Conversely, a reduced proportion of 6F results in enhanced membrane strength but entails the drawback of insufficient electrolyte retentivity and low electric conductivity. Thus, it is difficult for the copolymer to meet both the characteristics requirements (1) and (2). When 95/5 in 2F/6F mole ratio, the copolymer is still soluble in the electrolyte (mixture of equal portions of PC and EC), so that it is infeasible that a membrane swollen with an electrolyte under the conditions claimed in the U.S. Patent (6F/2F copolymer containing 8 to 25 wt. %, i.e., 4 to 12.5 mole %, of 6F) fulfill the foregoing requirement (2).
We have conducted intensive research and succeeded in fulfilling all the foregoing characteristics requirements (1) to (4) by using as a polymer gel electrolyte a polymer the molecule of which has a site having the function of retaining membrane strength and also a site having the function of being wettable with a liquid electrolyte.
The present invention provides a polymer gel electrolyte and a binder for use in lithium secondary batteries and lithium secondary batteries incorporating these components. More particularly, an object of the invention is to provide a polymer gel electrolyte having high ability to retain a liquid electrolyte and satisfactory membrane strength and a binder for use in lithium secondary batteries, and lithium secondary batteries comprising these components.